1. Field of the Invention
The present invention relates to methods of improving productivity of crop plants. In particular it relates to the application of compositions comprising tryptophan and/or tryptophan derivatives to plants.
2. Background of the Invention
Plant hormones (i.e., plant-growth regulators) play an important role in controlling plant growth and development. These compounds are useful for altering a plant's life processes or structure in some beneficial way so as to enhance yield, improve quality or facilitate harvesting. Growth-regulating compounds which are produced by plants are generally classified as one of four types of hormone: auxins, gibberellins, cytokinins and inhibitors. Many synthetic compounds mimic the activity characteristic of natural plant-growth regulators.
One class of plant hormone, auxins, and their synthetic mimics are of particular interest. Auxin-like activity is known to affect a number of plant processes, such as cell division, rooting at the basal end of shoots, shoot elongation, apical dominance, phototropic responses and control of abscission of organs such as buds, flowers, fruits, leaves and the like. Plant-growth regulators with auxin-like activity comprise an important class of chemicals for use in agriculture. As of 1994, there were approximately 29 compounds with auxin-like activity approved for agricultural use worldwide. Of these compounds, 21 were approved for use in the United States (KIRK-OTHMER ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY, 4th Ed., Kroschwitz et al., Eds., John Wiley & Sons, New York, 1994). A particularly widely used synthetic auxin is 2,4-dichlorophenoxyacetic acid (2,4-D). Among its many uses, 2,4-D is sprayed on the foliage of citrus trees in California and Florida (also in citrus growing countries such as Israel, Spain, Morocco, South Africa, etc.) to prevent preharvest fruit drop and to increase fruit size.
The agricultural application of exogenous chemicals to food crops is coming under increased scrutiny by many segments of society including the agricultural industry, advocates for agricultural laborers, environmental groups and consumers. In the United States, agricultural industry concerns stem from the fact that plant growth regulators must be officially registered with the Environmental Protection Agency (EPA) before they can be used or sold. Additionally, as plant-growth regulators are often applied closer to harvest than are pesticides, the actual practical requirements for their safety are more stringent.
The screening process associated with the official registration is both time-consuming and expensive. This process includes evaluation of a plant-growth regulator's safety hazards to humans, the environment and nontarget species. Further, acute and chronic toxicity must be determined. The agricultural industry shoulders a portion of the costs of the preregistration program in higher prices. Further, the industry typically bears part of the financial burden for reregistering the compound for a particular use. For example, the California citrus industry paid approximately two million dollars to effect the reregistration of 2,4-D as a preharvest fruit drop inhibitor. In addition to these financial concerns, the potential toxicity of synthetic plant-growth regulators raises additional concerns regarding the safety of their use.
The concerns of environmental groups, advocates for agricultural laborers and consumers arise from the potential toxicity of plant-growth regulators. For example, auxin mimics such as 2,4-D and related phenoxy acids have moderately acute toxicity and are moderate in their local effects upon the skin or eyes (Gehring et al. Ecol. Bull. (Stockholm) 27:122 (1978)). Results of cytogenic studies in Sweden indicate that, in practice, 2,4-D constitutes a cytogenic hazard to man (Jenssen et al. Chem. Biol. Interaction 14:291 (1976)). Additionally, 2,4-D has been found to exhibit central nervous system toxicity (Elo et al. Acta Pharmacol. Toxicol. 41:280 (1977)).
In light of their utility in preventing preharvest fruit drop and increasing fruit size, coupled with the expense of registration, reregistration and the potential toxicity of auxin mimics, alternatives to the use of synthetic auxins are being actively sought.
The application to plants and soils of natural auxins and natural auxin precursors is a particularly promising alternative to the use of synthetic auxin mimics. For instance, L-tryptophan has been reported to serve as precursor for the microbial formation of indole-3-acetic acid (IAA) (see, e.g., Arshad and Frankenberger Plant Soil 133:1-8 (1991)). Further, the synthesis of IAA upon application of tryptophan to soil has been shown to promote plant growth. For instance, growth of Douglas fir was increased by application of tryptophan and inoculation with a fungus capable of producing IAA from tryptophan (Frankenberger and Poth Appl. Environ. Microbiol. 53:2908-2913 (1987)). When tryptophan was applied to soils under aseptic conditions (i.e., steam-sterilized soil), L-TRP conversion to IAA was not observed (Martens and Frankenberger Soil Science 155:263-271 (1993)). Thus, it was concluded that the conversion of tryptophan to IAA was a microbe-mediated process.
Whether intact plants, in the absence of the soil microorganisms, are capable of converting tryptophan to IAA remains an open question which numerous experiments have attempted to answer. In spite of preliminary evidence that excised segments of plant are capable of converting tryptophan to IAA (see, e.g., Kutacek and Kefeli Biologia Plantarum (Praha) 12:145-158 (1970) and Bertling and Lovatt, Annual Meeting of the American Society for Horticultural Science, Aug. 7-10, 1994, HortScience 29, Abstract #752 (1994)), the question still remains whether the delivery to intact plants of tryptophan would result in auxins being synthesized to an extent sufficient to give rise to an auxin response in the plant.
When tryptophan is administered to intact, growing plants, yield data points to an absence of uptake or conversion of tryptophan. For example, in an experiment with radish plants which compared the effects of the foliar application of tryptophan to whole plants with the application of tryptophan to the soil, the application of tryptophan to the soil was found to promote radish growth. In contrast, the foliar application of tryptophan in dosage amounts between 10.sup.-2 M and 10.sup.-10 M had no effect on the growth of the radishes. Thus, it was concluded that tryptophan was not taken up by intact leaves and that the site of entry for tryptophan was through the soil and root (Frankenberger et al. Plant Soil 129:235-242 (1990).
A need exists for inexpensive, non-toxic and effective methods and compositions for improving crop productivity. If such methods and compositions were also as effective as the currently used treatment of plants with 2,4D they would indeed represent a significant advance over the current state of the art. Quite surprisingly, the present invention provides such methods and compositions.